2000 Ford Expedition Engine Hesitates

i'm not the original owner so i'm not sure what kind of thermostat was in the vechile. I recently replaced my thermostat and now when the vechicle gets to the new hotter operating tempature the engine is running rough when in gear. I first thought radiator fluid on the plugs or on the coil, but enough time has passed, (two days) and the truck has been driven a few times without getting to the full tempature, that any fluid should have evaporated. Next I thought head gasket but no oil in the radiator fluid or radistor fluid in the oil. Please help